Early in the Cybermen‘s history, before they froze themselves on Telos, some variants had crystalline eyes and antennae through which they received commands. These Cybermats resembled oversized metallic silverfish. They had segmented bodies with cilia-like tactile sensor probes along the bases of their heads. Some, though not all, Cybermats of this kind could fit in the palm of a humanoid hand. (The Tomb of the Cybermen)

In the 21st century, when the Cybermen attempted to infiltrate Space Station W3, they had photoreceptors for eyes rather than crystal-based eyes, spines down their backs and lacked the antennae. Otherwise, they looked similar to the kind seen on Telos. (The Wheel in Space)

Also in the 21st century, a type of Cybermat that had been buried beneath Colchester could drain electrical energy from the lights in town. This Cybermat had black, teardrop-eyes like Cybermen and an opening mouth with organic teeth. (Closing Time)

When the Cybermen infiltrated Nerva Beacon in a plot to destroy Voga, the Cybermats had segmented, eyeless, flat worm-like bodies without cilia. They had needles which injected poison into humans, but they could also inject gold dust into Cybermen. (Revenge of the Cybermen)

Another variant was a sleeker and more high-tech version of the Telosian Cybermats. Unlike the Telos version, these Cybermats were purposed with biting humans and infecting them with a virus, much like the Cybermats At The Nerva Beacon. However, instead of being deadly, the virus mutated flesh into metal and the victims became Cyberslaves. (Blood of the Cybermen)

Cybermats shared the Cybermen‘s weakness for gold, but it was possible for gold to be placed inside them without damaging their systems and the gold could be injected into Cybermen. (Revenge of the Cybermen) They could also be disabled with electric charges, (Revenge of the Cybermen) although this weakness seems to have been eliminated later; one was used to siphon electricity from power sources instead. The same model proved vulnerable to the sonic screwdriver, which was able to either stun or fry the Cybermat memory. (Closing Time) They could also be disabled with a sonic pulse, using an amplifier. (Blood of the Cybermen)

History

The Cybermats originated on Mondas during the earliest phases of Cyberman history, when most of the planet had not yet been converted. They were kept as pets. (Spare Parts)

The Cyber-tombs on Telos had tunnels for the Cybermats to move from the tombs to the main control rooms. (Revenge of the Cybermen) Cybermats were also seen on the surface of Telos. (The Doctor and the Dalek)

On GSO Arctic Drilling Station in 2010, most of the crew had been converted into Cyberslaves by the Cybermat’s nanovirus. The Eleventh Doctor repelled the Cybermats with a specific frequency on a sonic amplifier device and found a cure for the nanovirus to treat Chisholm. (Blood of the Cybermen)

In 2011, a group of Cybermen who had crashed on Earth centuries before were able to make their way to the surface. A Cybermat was used to drain electrical energy for the crashed Cybership so they could begin full Cyber Conversions. One was named “Bitey” by the Eleventh Doctor for its organic mouth. The Doctor reprogrammed it to drain power from the Cybership, but a Cyberman</a stepped on Bitey, destroying it. (Closing Time)

The Cybermats were used to guard the Cyberfactory when the Ninth Cyber Legion invaded London in 2012. The Cybermats were destroyed with the Cyberfactory. (The Eternity Clock)

Cybermen from the 30th century who travelled back to 1940 also employed Cybermats and harvested humans to make more Of them. (Illegal Alien)

The Cybermats appeared to have something to do with the conversion process. (Dreadnought)

In 2503, a Cybermat smuggled itself aboard the Vanguard and began sabotaging its systems. Vol shot it, disabling its weapons, but the Cybermen reprogrammed it to take control of the crew’s minds. (Sword of Orion)

The Master once forced the Graak to steal a disabled Cybermat. (Destiny of the Doctors)

When the Cybermen allies with Rassilon to change history, a swarm of Cybermats covered the Doctor’s TARDIS when the Eighth Doctor stepped out. (Supremacy of the Cybermen)

Cybermats were used by the Cybermen created by the Master on the Mondasian colony ship. They continued to appear every spring by the time Nardole died, 491 years after he stayed behind to defend the colony, though by that time the Mondasians had “basically sorted” their conflict with the Cybermen. (Twice Upon a Time)

Behind the scenes

Wood lice inspired the creation and look of the Cybermats. In their first appearances in The Tomb of the Cybermen and The Wheel in Space, they were realised in the studio by a variety of techniques, including clockwork and radio-controlled models. In some shots they were pulled by wires or simply pushed into view. Shots of the Cybermats leaping onto their victims were achieved by pulling the Cybermat away with wires and playing the film backwards.
Cybermat effects in Revenge of the Cybermen used a combination of strings, puppetry and CSO. The Cybermats (or rather the single Cybermat which appeared) went through a radical re-design and resembled a cybernetic silverfish.
According to The Brilliant Book 2011, a reference book that is not considered a valid source on this wiki, on 30 November 1879 at Blenheim Palace, the Fourth Doctor was hunting Cybermats while posing as a Punch and Judy man for Winston Churchill.
The Cybermats are minor villains in LEGO Dimensions.